Unlock extra options:
Complete the game on any difficulty level with rank D or above to enable the versus option on the main menu.
Complete the game a second time to unlock the remaining two orbital frames and two battle levels in versus mode.
Complete the game 50 times on one player versus mode to get Anubis as a playable frame.

Alternate ending sequences:
Earn an "A: rank in all S.O.S. calls. New music will play during the ending sequence. Intentionally do badly during the game by destroying all buildings and killing all survivors during the various missions. This includes completely destroying the colony shaft during the last SOS mission. Defeat Tyrant and go to the Mountain 1 level to view the worst ending.

Alternate ending music:
Get an "A: rank in all the S.O.S. calls. New music will play during the ending sequence